Isometrically mounted and electrically stimulated strips of rabbit mesotubarium superius muscles were subjected to sinusoidal length perturbations (delta L) of variable amplitude (0-1% of Lmax) at constant frequency (80 Hz). In both the plateau phase of long-maintained (steady-state) contractions and in brief contraction-relaxation cycles the force perturbation amplitude (delta F) response was directly proportional to delta L (and to the developed force, F) for delta L values less than approximately 0.30% of Lmax. At larger delta L values the magnitude of the delta F response deviated downward from a linear relationship. Statistical analysis of the relationship characterized a "deviation point," the value of delta F and delta L beyond which the delta F response departed from linearity. The value of delta F at the deviation point depended strongly on the developed force, whether it was varied by changing muscle rest length, stimulus strength, or time during a phasic contraction-relaxation cycle. The value of delta L at the deviation point was independent of developed force. Restricting the length of muscle subjected to delta L (while not changing cellular dimensions) showed that the slope of delta F/delta L relationship (the active stiffness) depended inversely on the length of muscle in the system. These findings are consistent with a model in which the active elasticity arises from cross bridges that are borne on myofilaments and that both generate and transmit the muscle force.